linux-kernel - Re: [PATCH v3] x86,mm: only trim the mm

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Message-ID: <43835b8c-53cd-428f-a46f-554bfa5c2cdd@efficios.com>
Date: Wed, 4 Dec 2024 15:19:46 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Rik van Riel <riel@...riel.com>, Oliver Sang <oliver.sang@...el.com>
Cc: oe-lkp@...ts.linux.dev, lkp@...el.com, linux-kernel@...r.kernel.org,
 x86@...nel.org, Ingo Molnar <mingo@...nel.org>,
 Dave Hansen <dave.hansen@...el.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v3] x86,mm: only trim the mm_cpumask once a second

On 2024-12-04 11:56, Rik van Riel wrote:
[...]
> 
> Signed-off-by: Rik van Riel <riel@...riel.com>
> Reported-by: kernel test roboto <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202411282207.6bd28eae-lkp@intel.com/
> ---
>   arch/x86/include/asm/mmu.h         |  2 ++
>   arch/x86/include/asm/mmu_context.h |  1 +
>   arch/x86/include/asm/tlbflush.h    |  1 +
>   arch/x86/mm/tlb.c                  | 35 +++++++++++++++++++++++++++---
>   4 files changed, 36 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
> index ce4677b8b735..3b496cdcb74b 100644
> --- a/arch/x86/include/asm/mmu.h
> +++ b/arch/x86/include/asm/mmu.h
> @@ -37,6 +37,8 @@ typedef struct {
>   	 */
>   	atomic64_t tlb_gen;
>   
> +	unsigned long next_trim_cpumask;
> +
>   #ifdef CONFIG_MODIFY_LDT_SYSCALL
>   	struct rw_semaphore	ldt_usr_sem;
>   	struct ldt_struct	*ldt;
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index 2886cb668d7f..795fdd53bd0a 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -151,6 +151,7 @@ static inline int init_new_context(struct task_struct *tsk,
>   
>   	mm->context.ctx_id = atomic64_inc_return(&last_mm_ctx_id);
>   	atomic64_set(&mm->context.tlb_gen, 0);
> +	mm->context.next_trim_cpumask = jiffies + HZ;
>   
>   #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
>   	if (cpu_feature_enabled(X86_FEATURE_OSPKE)) {
> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index 69e79fff41b8..02fc2aa06e9e 100644
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -222,6 +222,7 @@ struct flush_tlb_info {
>   	unsigned int		initiating_cpu;
>   	u8			stride_shift;
>   	u8			freed_tables;
> +	u8			trim_cpumask;
>   };
>   
>   void flush_tlb_local(void);
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 1aac4fa90d3d..a758143afa01 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -892,9 +892,36 @@ static void flush_tlb_func(void *info)
>   			nr_invalidate);
>   }
>   
> -static bool tlb_is_not_lazy(int cpu, void *data)
> +static bool should_flush_tlb(int cpu, void *data)
>   {
> -	return !per_cpu(cpu_tlbstate_shared.is_lazy, cpu);
> +	struct flush_tlb_info *info = data;
> +
> +	/* Lazy TLB will get flushed at the next context switch. */
> +	if (per_cpu(cpu_tlbstate_shared.is_lazy, cpu))
> +		return false;
> +
> +	/* No mm means kernel memory flush. */
> +	if (!info->mm)
> +		return true;
> +
> +	/* The target mm is loaded, and the CPU is not lazy. */
> +	if (per_cpu(cpu_tlbstate.loaded_mm, cpu) == info->mm)
> +		return true;
> +
> +	/* In cpumask, but not the loaded mm? Periodically remove by flushing. */
> +	if (info->trim_cpumask)
> +		return true;
> +
> +	return false;
> +}
> +
> +static bool should_trim_cpumask(struct mm_struct *mm)
> +{
> +	if (time_after(jiffies, mm->context.next_trim_cpumask)) {
> +		mm->context.next_trim_cpumask = jiffies + HZ;

AFAIU this should_trim_cpumask can be called from many cpus
concurrently for a given mm, so we'd want READ_ONCE/WRITE_ONCE
on the next_trim_cpumask.

Thanks,

Mathieu

> +		return true;
> +	}
> +	return false;
>   }
>   
>   DEFINE_PER_CPU_SHARED_ALIGNED(struct tlb_state_shared, cpu_tlbstate_shared);
> @@ -928,7 +955,7 @@ STATIC_NOPV void native_flush_tlb_multi(const struct cpumask *cpumask,
>   	if (info->freed_tables)
>   		on_each_cpu_mask(cpumask, flush_tlb_func, (void *)info, true);
>   	else
> -		on_each_cpu_cond_mask(tlb_is_not_lazy, flush_tlb_func,
> +		on_each_cpu_cond_mask(should_flush_tlb, flush_tlb_func,
>   				(void *)info, 1, cpumask);
>   }
>   
> @@ -979,6 +1006,7 @@ static struct flush_tlb_info *get_flush_tlb_info(struct mm_struct *mm,
>   	info->freed_tables	= freed_tables;
>   	info->new_tlb_gen	= new_tlb_gen;
>   	info->initiating_cpu	= smp_processor_id();
> +	info->trim_cpumask	= 0;
>   
>   	return info;
>   }
> @@ -1021,6 +1049,7 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
>   	 * flush_tlb_func_local() directly in this case.
>   	 */
>   	if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids) {
> +		info->trim_cpumask = should_trim_cpumask(mm);
>   		flush_tlb_multi(mm_cpumask(mm), info);
>   	} else if (mm == this_cpu_read(cpu_tlbstate.loaded_mm)) {
>   		lockdep_assert_irqs_enabled();

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com